In a digital subscriber line (xDSL) communication system, loop imbalance with respect to ground results in unwanted external noise being coupled into differential mode transmissions and being superimposed onto the desired xDSL signal transmitted on a twisted pair. Such noise sources include far-end cross talk (FEXT), near-end crosstalk (NEXT), radio frequency interface (RFI), impulse noise, power line communication (PLC) interference, and so on. The ITU-T standard defines requirements relating to longitudinal conversion loss (LCL) for terminating equipment to be greater than 38 dB, in order for the equipment (whether a customer premises equipment (CPE) or central office (CO) transceiver) not to be the limiting factor in the system.
However, the ITU-T standard currently does not define a loop imbalance requirement of twisted pairs as part of the G.--LT (single end loop test (SELT), dual end loop test (DELT), metallic loop test (MELT)) or G.993.2/G.992.3 standards that allows one to qualify in-situ the loop balance characteristics and its impact on the DSL bands being used. The ITU-T standard also does not define the values of the common mode impedance of the terminating equipment required to reduce propagation of common mode signals.